1. Field of the Invention
The present invention relates to a pulse width modulator, and more particularly to a pulse width modulator having an amplitude-adjustable triangle-wave generator for modifying the amplitude of the triangle wave, changing the width of the controlling signal and enhancing the transient response of the loading system.
2. Description of the Related Art
Pulse width modulation (PWM) is a traditional method for controlling the feedback circuit, such as the boosting circuit, the buck circuit, the push-pull circuit, half-bridge circuit or full-bridge circuit. In the prior art technology, error amplifiers and triangular-wave generators for generating fixed-amplitude triangle wave as to generate a pulse according to the error variations.
FIG. 5 is a block circuit diagram showing a conventional pulse width modulator of a loading system. The loading system 50 comprises: a pulse width modulator 500, a switch 550 and a loading circuit 560. The switch 550 is coupled to a power terminal, the pulse width modulator 500 and a ground terminal. The input terminal 562 of the loading circuit 560 is coupled to the power terminal. The output terminal 564 of the loading circuit 560 is coupled to the ground terminal. The pulse width modulator 500 is coupled to the output terminal 564 of the loading circuit 560.
The pulse width modulator 500 comprises: a triangle-wave generator 510, an error amplifier 520, a comparator 530 and a driving circuit 540. A negative terminal of the error amplifier 520 is coupled to the output terminal 564 of the loading circuit 560 for receiving the feedback voltage therefrom. The positive terminal of the error amplifier 520 is coupled to a source of reference voltage. The positive terminal of the comparator 530 is coupled to the error amplifier 520, the negative terminal of the comparator 530 is coupled to the triangle-wave generator 510, and the output terminal of the comparator 530 is coupled to the driving circuit 540. FIG. 6 is a schematic drawing showing a conventional triangle-wave generator outputting the triangle wave, the error signal and the pulse controlling signal. The error amplifier 520 performs an error operation of the feedback voltage and the reference voltage for outputting an error signal 604 to the comparator 530. After the pulse width modulator 500 is turned on, the triangle-wave generator 510 outputs a fixed-amplitude triangle wave 602 to the comparator 530. The comparator 530 compares the error signal 604 and the fixed-amplitude triangle wave 602 for outputting the driving signal. Finally, the driving circuit 540 outputs a pulse controlling signal 606.
In the prior art loading system 50, the switch 550 controls the power terminal according to the pulse controlling signal from the comparator 530. When the switch 550 is turned on, the power is not transmitted to the loading circuit 560; if not, the power is transmitted to the loading circuit 560.
Accordingly, the conventional pulse width modulator have the following disadvantages: (1) In the conventional pulse width modulator, because the width of the pulse controlling signal is modulated by the error of the feedback voltage and the reference voltage, noises in the feedback voltage will interfere the width of the pulse controlling signal. (2) The conventional pulse width modulator generates a desired transient response for the loading system, but degrade the stability thereof. (3) The conventional pulse width modulator generates a desired stability of the loading system, but degrades the transient response thereof.